Implantable cardioverter defibrillators (ICDs) typically sense intracardiac EGM signals for detecting arrhythmias. Ventricular tachycardia (VT) and fibrillation (VF) are detected by analyzing cardiac intervals, sometimes in conjunction with EGM signal morphology analysis. When VT or VF is detected, the heart may be cardioverted or defibrillated using one or more high voltage shocks. A shock may be delivered immediately when the detected arrhythmia is considered to be a potentially lethal arrhythmia. A shock may be delivered after less aggressive pacing therapies are attempted first and are unsuccessful in terminating VT.
A shock is painful to the patient and uses considerable battery energy. The avoidance of unnecessary shocks is important in preventing undue pain to the patient and to conserve battery longevity of the implanted device. An unnecessary shock may be delivered, for example, when VT or VF is falsely detected due to oversensing of non-cardiac signals, T-wave oversensing, or when a supraventricular tachycardia (SVT) is falsely detected as VT or VF. A need remains, therefore, for reducing the number of unnecessary shocks delivered by ICDs.